Ethylbenzene is one of the aromatic hydrocarbons that can be obtained from naphtha pyrolysis or reformate. Reformate is an aromatic product obtained by the catalysed conversion of straight-run hydrocarbons boiling in the 70 to 190° C. range, such as straight-run naphtha. The reformate feedstock itself is obtained by fractionation or distillation of crude petroleum oil, its composition varying depending on the source of the crude oil, but generally having a low aromatics content. On conversion to reformate, the aromatics content is considerably increased and the resulting hydrocarbon mixture becomes highly desirable as a source of valuable chemical intermediates and as a component for gasoline. The principle components are a group of aromatics often referred to as BTX: benzene, toluene and the xylenes, including ethylbenzene. Other components may be present such as their hydrogenated homologues, e.g. cyclohexane.
Of the BTX group the most valuable components are benzene and the xylenes, and therefore BTX is often subjected to processing to increase the proportion of those two aromatics: hydrodealkylation of toluene to benzene and toluene disproportionation to benzene and xylenes. Within the xylenes, para-xylene is the most useful commodity and xylene isomerisation or transalkylation processes have been developed to increase the proportion of para-xylene.
A further process that the gasoline producer can utilize is the hydrodealkylation of ethylbenzene to benzene.
Generally, the gasoline producer will isolate BTX from the reformate stream, and then subject the BTX stream to xylene isomerisation with the aim of maximising the para-xylene component. Xylene isomerisation is a catalytic process. Some catalysts used in this process have the ability not just to isomerise xylenes but to simultaneously dealkylate the ethylbenzene component. Normally the para-xylene is then separated out to leave benzene, toluene (unless toluene conversion processes have already been applied) and the remaining mixed xylenes, including ethylbenzene. This BTX stream can either be converted by transalkylation to increase the yield of xylenes by contacting with a heavier hydrocarbon stream or can be converted by dealkylation to selectively eliminate ethylbenzene and to increase the yield of benzene, while allowing the xylenes to reach equilibrium concentrations. The latter process is the subject of the present invention.
In ethylbenzene dealkylation at this latter stage of BTX treatment, it is a primary concern to ensure not just a high degree of conversion to benzene but also to avoid xylene loss. Xylenes may typically be lost due to transalkylation, e.g. between benzene and xylene to give toluene, or by addition of hydrogen to form, for example, alkenes or alkanes.
EP2027917 describes an ethylbenzene dealkylation catalyst which is prepared by mixing at least 20% wt of ZSM-5 having a bulk silica to alumina ratio in the range of from 20 to 150 and being in its H+ form with at least 30% wt of a binder selected from silica, zirconia and titania, drying and calcining the extrudates obtained, incorporating platinum and tin into the calcined extrudates, and drying and calcining the metal containing composition thus obtained. It does not mention the para-xylene content of product obtained in a dealkylation process using such catalyst.
US20130197290 describes an ethylbenzene dealkylation catalyst which is prepared by mixing and extruding ZSM-5 having a bulk silica to alumina ratio in the range of from 20 to 150 with water, a silica source and an alkali metal salt, drying and calcining the extrudates obtained, subjecting the calcined extrudates to ion exchange to reduce the alkali metal content, drying the ion exchanged extrudates, incorporating platinum and tin into the dried extrudates and drying and calcining the metal containing composition thus obtained. The para-xylene content of product obtained in a dealkylation process using such catalyst has not been described.
US20160017238 describes a catalyst for converting solid biomass into fuel of specialty chemical products.
U.S. Pat. No. 4,511,547 describes a process for preparing crystalline aluminosilicate zeolite.
The article by Christensen et al.: “Mesoporous zeolite single crystal catalysts: Diffusion and catalysis in hierarchical zeolites”, Catalysis Today, Elsevier NL, vol. 128, no. 3-4, 30 Oct. 2007, pages 117-122, describes the use of mesoporous zeolite catalysts in alkylation of benzene with ethene. Analysis of the results obtained suggests that beneficial effect of mesopores also should be observed in the dealkylation of ethylbenzene. The para-xylene content of product obtained in a dealkylation process using such catalyst has not been described.